The major objectives of the proposed study are: 1. To determine the most common serotypes of S. pneumoniae which cause pneumonia, sepsis, and meningitis in patients with sickle cell diseases. 2. To determine if patients with sickle cell diseases (SS, SC, S thalassemia) and normal children can effectively opsonize their infecting strain of S. pneumoniae prior to, during, and after infection, and to determine if a rise in specific antibody titer is associated with the appearance of serum opsonizing activity. 3. To determine if peripheral leukocytes from patients with sickle cell diseases can effectively phagocytose and kill their infecting strain of S. pneumoniae during infection, and, if not, to determine if the functional leukocyte abnormality is corrected during recovery. 4. To determine if complement consumption occurs during pneumococcal infection or hematologic crisis in sickle cell disease. 5. To determine if opsonic or leukocyte abnormalities occur during hematologic crisis. The primary methods to be used are: 1. Isolation and serological identification of strains of S. pneumoniae. 2. Measurements of serum opsonic and leukocytic bactericidal activities for S. pneumoniae. 3. Measurements of levels and activities of components of the classical and alternative pathways of complement activation. 4. Chromatographic methods for isolation of IgG from normal human serum.